FUEL EFFECTS ON KNOCK, HEAT RELEASE AND CARS TEMPERATURES IN A SPARK-IGNITION ENGINE

Net heat release, knock characteristics and temperature were derived f rom in-cylinder pressure and end-gas CARS measurements for different f uels in a single-cylinder engine. The maximum net heat release rate re sulting from the final phase of autoignition is closely associated wit h knock intensity. Aromatic fuels have lower maximum heat release rate s and lower knock intensities than expected from their octane number w hen compared to paraffinic fuels; this is observed even when there is significant heating of the end-gas from pre-flame reactions. Leaner mi xtures have lower combustion rates so that pressure development is slo wed and hence ignition needs to be more advanced to get knock to occur as frequently as in a richer mixture. However, for a given frequency of knock occurrence, there is no significant difference in peak net he at release rates and hence in knock intensities for different mixture strengths.